Process for chemicals and energy recovery from waste liquors

ABSTRACT

Process for recovering chemicals and energy from cellulose waste liquors preferably black kraft liquor obtained in a paper pulp sulphate process, comprising the following, three distinct and separate steps: In the first step the concentrated black liquor is gasified in a pressurized gasification reactor by so called flash-pyrolysis at 700° to 1300° C., normally 800°-900° C., whereby an energy rich gas is produced, and in which the inorganic chemicals of the black liquor are contained in the form of molten suspended droplets, mainly comprising sodium carbonate and sodium sulphide. In the second step the gas from the gasification reactor is rapidly cooled through direct contact with water, and with green liquor, which is formed when the molten droplets and the hydrogen sulphide are dissolved in the quench liquid. The cooled gas subsequently passes through a scrubber. In the lower section of the scrubber the gas is washed with circulating green liquor, and in the upper section of the scrubber the gas is washed with sodium hydroxide (or carbonate) solution and water for complete removal of any remaining sulfur bearing components in the gas. In the third step the now sulfur and particulate-free gas is used as a fuel for generating steam and, if it should be economically justified, also for production of electric power.

TECHNICAL FIELD

The present invention relates to a process for recovering cookingchemicals and energy from cellulose waste liquors obtained in the pulpand paper industry.

The object of the present invention is to recover chemicals and energyfrom waste liquors preferably black kraft liquors obtained in the pulpand paper industry, and in particular the pulp and paper industryproducing according to the sulphate method, whereby a more specificobject is to eliminate prior art known problems in the use of huge andtechnically and operationally complicated combustion furnaces in therecovery of chemicals and energy, and polluting the air with primarilysulfur containing exhaust gases, i e to recover substantially allchemicals and energy present in waste liquors in an efficient andenvironmentally attractive way.

The term waste liquors used herein means preferably black kraft liquorbut includes waste liquors obtained from the sodium bisulfite and sodiumcarbonate processes as well. In the following the term waste liquors andblack kraft liquor are used synonymously.

BACKGROUND OF THE INVENTION

When producing paper pulp a digestion liquor called black kraft liquoris obtained, which liquor besides numerous solubilized organic materialsalso contains the chemicals used for digesting the wooden raw material,i e sodium hydroxide and sodium sulphide, now mainly in the form ofsodium carbonate and sodium sulphate. The black kraft liquor isgenerally concentrated before combustion for recovery of the energycontent of the organic matter and for recovery of the cooking chemicalsas so called green liquor.

Different processes have been proposed for recovering the energy andcooking chemicals, whereby the Tomlinson process is one, using a hugecombustion furnace with steam raising heat recovery. Another proposedprocess is the so called cyclone combustion process, in which pyrolysisand combustion of the organic material takes place in a cyclone chamber.This latter process has, however, not gained any commercial success sofar, and has only been tested in pilot plant.

DESCRIPTION OF THE PRESENT INVENTION

It has now by this invention been found possible to eliminate prior artknown drawbacks, and to be able to recover essentially all of thechemicals and energy present in the black kraft liquor, i e to minimizelosses, by a novel and elegant process, which is characterized asfollows.

(i) introduction of the black kraft liquor into a pressurizedgasification reactor, while being atomized and being subjected to aflash-pyrolysis, while mainly forming CO, CO₂, H₂, H₂ S, Na₂ CO₃ and Na₂S.

(ii) introduction of oxygen or oxygen containing gas, preferably at somedistance from the point of introducing black kraft liquor into saidreactor, in order to support the endothermal pyrolysis reactions bycombustion of a part of the gas formed by the pyrolysis of the organicmatter present in the black kraft liquor. Optionally introducing asupport fuel such as oil or gas to said reactor, if needed, formaintaining the heat balance around said reactor.

(iii) maintaining in said reactor a temperature of 700° to 1300° C.,preferably 800° to 1000° C. to achieve complete gasification of theorganic content of the black liquor and to form droplets of meltedinorganic chemicals, mainly comprising Na₂ CO₃ and Na₂ S.

(iv) transporting said melt of chemicals out of said reactor by means ofthe rapid gas flow obtained during said gasification and cooling saidgas and melt of chemicals, mainly in the form of droplets, in an aqueousquench solution.

(v) washing said gas with regard to its contents of solubilizableinorganid chemicals using an alkaline solution such as sodiumcarbonate/sodium sulphide solution (green liquor), sodium hydroxide (orcarbonate) solution, and water in adequate order for dissolving saidinorganic chemicals and as complete absorption as possible of the sulfurbearing compounds in the pyrolysis gas.

(vi) removing remaining gas and using it as an exceptionally clean fuelfor generation of steam and/or electric power, preferably in a so calledcombined gas turbine/steam turbine cycle.

Further characteristics are evident from the accompanying claims.

The key feature of the novel concept described herein is that therecovery of chemicals and energy from the concentrated kraft blackliquor is carried out in three distinct and separate steps.

In the first step the concentrated black liquor is gasified in apressurized gasification reactor by so called flash-pyrolysis at 700° to1300° C., preferably 800° to 1000° C., normally 800° to 900° C., wherebyan energy rich gas is produced and in which the inorganic chemicals ofthe black liquor are contained in the form of molten suspended droplets,mainly comprising sodium carbonate and sodium sulfide.

In the second step the gas from the gasification reactor is rapidlycooled through direct contact with water, and with green liquor, whichis formed when the molten droplets and the hydrogen sulphide aredissolved in the quench liquid. The cooled gas subsequently passesthrough a scrubber. In the lower section of the scrubber the gas iswashed with circulating green liquor and in the upper section of thescrubber the gas is washed with a sodium hydroxide (or carbonate)solution and water for complete removal of any remaining sulfur bearingcomponents in the gas.

In the third step the now sulfur and particulates-free gas is used as afuel for generating steam and, if it should be economically justified,also for production of electric power.

By conducting the first steps of the process described above at elevatedpressure, as at a level of about 3 to 150 bars, preferably 10 to 100bars, more preferably 30 to 50 bars, the following major advantages aregained, viz.

Compact process equipment with high throughput capacity, which resultsin low investment cost per unit of kraft liquor throughput. The volumeof the reactor can be reduced to less than 1/100 of what is previouslyknown and required.

Recovery of the physical heat content of the gas as proces steam whencooling the gas from the gasification temperature of 800°-900° C. to thesaturation temperature at the selected pressure, which at e g 40 barsamounts to about 200° C. At this temperature steam at 3-8 bars pressurecan be generated when cooling the circulating green liquor and whencooling the gas and condensing its water vapor content downstream of thescrubbing tower.

Utilization of the gas for very efficient production of steam andelectricity, e g in so called combined gas turbine/steam turbine cycle,whereby an electricity yield of around 50% can be achieved.

If, despite the rigorous scrubbing of the gas, some sulfur compoundswould slip through the scrubbing tower, these compounds are completelyconverted to SO₂ in the subsequent combustion step, thereby eliminatingbad smelling H₂ S and mercaptane emissions with the exhaust gases.

Theoretical investigations (Henry, R. E., and Fauske, H. K., Trans.Asme., J. HT Trans. 101(2):280, May 1979) show that melt/waterexplosions are avoided if the pressure is higher than 9 bars.

The invention will be described in more detail in the following withreference to the accompanying drawing which shows a flow sheet of a setup of equipment for carrying out the invention.

1 denotes a gasification reactor lined with refractory material. Thereactor 1 is provided with an inlet 3 for black kraft liquor. A burner 4is arranged for optional supply of heat. The liquor is introduced via aline 5, and atomizing gas (steam) is supplied via a line 6. Oxygen oroxygen containing gas is supplied via a line 7 connected to the reactor1 via the burner 4. The reactor chamber opens downwardly into a quenchwater chamber 8 from which a bottom outlet 9 leads. This outlet 9 passesa steam generator 10, in which heat exchange takes place. The steamgenerated is used as a process steam used elsewhere, and is removed viaa line 11. The outlet 9 containing green liquor passes green liquor tothe pulp process 14 via a heat exchanger 12 for heating fresh supplywater coming in, or passes green liquor to a scrubber 15, to the lowerpart thereof, and/or passes green liquor in return to the quench chamber8. An outlet 16 from the quench chamber 8 leads the gas, mainly CO, H₂,H₂ S, CO₂ and H₂ O to the scrubber 15. Purified gas leaves the scrubber15 via the top thereof via a line 17 leading via an additional steamgenerator 18 to a condensor 19 for eliminating water present in the gaswhich is fed to a combustion chamber 20 and in which CO and H₂ arecombusted to CO₂ and H₂ O by means of air added via a line 22. Exhaustgases generated are transferred to a gas turbine 23 where the gas isexpanded to slightly above atmospheric pressure and said gas is thencooled in a waste heat boiler 21 to produce superheated high pressuresteam connected to a back pressure steam turbine 24. Said turbines 23and 24 are connected to a generator 25 for production of electricity. Achimney 26, or stack, is finally taking care of remaining exhaust gas(CO₂ and H₂ O). Steam produced in the steam generator 18 is used asprocess steam, and boiler feed water is thereby supplied via a line 28,and is normally in the form of steam condensate. Green liquor is fed viaa line 29 to the lower part of the scrubber 15. Water provided with Na₂CO₃ or NaOH 36 is supplied to the lower upper third of the scrubber 15via a line 30. The water used is preferably the condensate from thecondenser 19. On the top of the scrubber 15 water, preferably in form ofcondensate from the condenser 19, is supplied via a line 31. Combustiongas is if so required shunted away from the line 17 to supply the burner3. Condensate water from the condenser 19 can also be supplied to thequench liquid chamber 8 via a line 33. Green liquor obtained in thescrubber 15 is recirculated to the quench chamber 8 via a line 34. Theoutlet 35 from the steam turbine is arranged to supply process steam 11.

Step 1--Gasification

The concentrated kraft black liquor is injected through an atomizingnozzle 2 located at the top of the refractory lined gasificationreactor 1. Depending on the type of atomizing nozzle 3, steam, nitrogen(or other inert gas) or pressure can be used to achive the requireddispersion of the black liquor. The placement of the atomizingnozzle(-s) can, however, be varied, as well, and can advantageously beplaced at some distance from the top of the reactor, e.g., through thevertical jacket wall, and can as indicated consist of several nozzle, aswell.

Air, oxygen or oxygen enriched air can be used as gasifcaton medium.When sprayed into the reactor the droplets of the kraft liquor undergoso called flash-pyrolysis, which is an endothermal process. In order toprovide the required heat of reaction and to maintain the temperature inthe reactor as given above which is necessary for complete conversion ofthe organic content of the black liquor to gas, oxygen, or air is addedto the gasification reactor 1. The oxygen, or air, must be added in sucha way that the hydrogen and carbon monoxide formed during the pyrolysisof the black liquor droplets react with the oxygen before the latter hashad the time to come in contact with the pyrolysed droplets and oxidizetheir content of sulfides to sulfate or thio-sulfate. In order tomaintain the heat balance around the gasification reator 1 somesupplemantary combustion of e g oil or recirculated pyrolysis gas mightbe required. Such support-firing will, however, always be requiredduring start-up and shut-down operations.

In order to achieve complete conversion of the organic substance to gas,mainly consisting of hydrogen, carbon monoxide, carbone dioxide, watervapor and hydrogen sulfide (and possibly also nitrogen) a temperature inthe range of 800°-1000° C. is required at the outlet of the gasificationreactor. At this temperature the sulfur and sodium is contained in themolten droplets mainly in the form af sodium carbonate and sodiumsulfide. These droplets will partly hit the reactor walls and form adown-ward flowing film, which with the aid of the high gas velocity istransferred into the quench chamber 8 installed below the gasifier.

The refractory lined reactor 1 can be equipped with built-in coolingcoils (not shown), whereby the wall temperature is controlled in such away that a protective layer of solidified melt is formed on therefractory wall. If, on the other hand, it should be possible to find arefractory material, which has adequate resistance to the melt, areactor design without cooling coils can be choosen.

Step 2--Gas cooling and scrubbing/recovery of chemicals

At the outlet 16 from the gasification reactor the gas with itsentrained molten droplets is rapidly cooled through direct contact withwater and the green liquor, which is formed when the droplets and thehydrogen sulfide are dissolved in the quench water. The main part of thecooling takes place as a result of evaporation of part or all of thequench liquid. The contact between the gas and the quench liquid can beaccomplished in different ways, e g by passing the gas through adip-tube of a water-trap, by injecting the cooling liquid into the gasstream or by a combination of these methods.

The temperature in the cooling and scrubbing step is governed by theselected operating pressure and is related to the temperature ofsaturated steam at this pressure. At an operation pressure of e g 40bars an equilibrium temperatur in the order of 200° C. can be expectedin the cooling/scrubbing step.

In the scrubber 15 down-stream of the quench chamber 8 the gas is washedin the bottom section with circulating green liquor. In the uppersection of the scrubber the gas is first washed with a sodium carbonate,or sodium hydroxide solution and finally with water for complete removalof any remaining sulfur bearing compounds in the gas. A regenerativeprocess for absorption/desorption of the gaseous sulfur compounds likean amine wash may constitute an alternative for producing a sulfur freegas.

As a result of the high operating pressure and the corresponding hightemperature the heat absorbed in the cooling and scrubbing circuit canbe recovered in steam generators 10, 18 by condensation of the waterevaporated in the quench chamber 8 (heat exchanger 18) and by extractingphysical heat from the circulating green liquor (heat exchanger 10). Thesteam can be generated at a pressure of approximately 5 bars which makesit suitable as process steam within the pulp and paper plant. In orderto maintain the water balance of the total system, make-up water (e gcondensate from the black liquor evaporation) is added to compensate forthe water withdrawn from the system as green liquor. For heat recoverypurposes the make-up water should be preheated in the heat exchanger 12against the green liquor draw-off.

For final adjustment of the desired water vapor content of the gas fromthe scrubber 15 the gas is cooled down-stream of the steam generator 18with cooling water in a heat exchanger. The condensate is routed back tothe quench chamber 8 and to the scrubber 15.

Step 3--Energy recovery

Although some heat recovery took place in the preceeding step in thesteam generators 10, 18, the major share of the energy recovery isperformed in the third step, where the chemical energy of the gas isutilized. The sulfur and essentially water free gas, which is nowavailable at about 35 bars and say 100° C., constitutes an excellentfuel for generation of steam and/or electricity. Due to the extremecleanliness of the gas the steam and electricity production facilities20, 23, 24 can be based on simple and low cost designs and yield veryhigh energy efficiency. The most cost effective way of utilizing the gasis to generate electricity and back pressure steam in a combined gasturbine/steam turbine cycle, which operates as follows.

The gas is combusted with air under pressure in a combustion chamber 20from where the exhaust gas passes a gas turbine 23 connected to aelectric generator 25. The exhaust gas from the turbine 23 leaving athigh temperature, then passes as waste heat boiler 28 which generateshigh pressure superheated steam. The steam is used to drive a backpressure steam turbine 24 connected on the same shaft as the abovementioned gas turbine 23, thereby increasing the electricity production.The back pressure of the steam turbine 24 is selected to suit the use ofthe removed steam as process steam. By such an arrangement an overallenergy efficiency of 92-93% can be achieved and of the energy output upto 50% can be obtained as electricity. This results in a significantlyhigher overall yield of electricity from the waste liquor energy than ispresently obtained in a conventional process.

I claim:
 1. A process for recovering chemicals and energy from cellulosetreatment waste liquors containing organic matter comprising:(i)introducing said waste liquor into a pressurized gasification reactorwhile being atomized and subjected to flash-pyrolysis thereby formingCO, CO₂, H₂, H₂ S, Na₂ CO₃ and Na₂ S; (ii) introducing a gas selectedfrom the group consisting of oxygen or an oxygen containing gas intosaid reactor in order to support endothermal pyrolysis reactions bycombustion of a part of the gas formed by the pyrolysis of the organicmatter present in said liquor; (iii) maintaining a temperature of atleast 800° to 1300° C. and a pressure from 3 to 150 bars in said reactorto achieve complete gasification of the organic content of the wasteliquor and to form droplets of melted inorganic chemicals comprising Na₂CO₃ and Na₂ S; (iv) transporting said melt of chemicals out of saidreactor into a quenching chamber by means of high velocity gas flowobtained during said gasification and cooling said gas and melt ofchemicals through direct contact with an aqueous quench liquid; (v)washing the cooled gas from step (iv) with an alkaline solution in orderto dissolve the chemical content thereof and in order to remove thesulfur bearing compounds from the pyrolysis gas; and (vi) removing saidremaining gas and using it as a fuel for generating power.
 2. A processaccording to claim 1, wherein the temperature of the reactor ismaintained between 800°-1000° C.
 3. A process according to claim 1,wherein the temperature in the gas cleaning and energy recovering stepsof the process are maintained from 120° to 300° C.
 4. A processaccording to claim 1, wherein the alkaline washing solution comprisessodium carbonate, sodium hydroxide or a mixture of sodium carbonate andsodium sulfide.
 5. A process according claim 1, wherein said cooled gasis finally washed with water.
 6. A process according to claim 1, whereinsteam and electricity are generated in a combined gas and steam turbinecycle.
 7. A process according to claim 1, wherein a support fuel isintroduced into said reactor when needed in order to maintain the heatbalance around said reactor.
 8. A process according to claim 7, whereinsaid fuel is recirculated pyrolysis gas from the process.
 9. A processaccording to claim 1, wherein the introduction of oxygen or oxygencontaining gas takes place at some distance from the point ofintroducing the waste liquor into said reactor.
 10. A process accordingto claim 2, wherein the temperature is maintained between 800°-900° C.11. A process according to claim 1, wherein the pressure in the reactoris from 10 to 100 bars.
 12. A process according to claim 11, wherein thepressure in the reactor is from 30 to 50 bars.
 13. A process accordingto claim 3, wherein the temperature in the gas cleaning and energyrecovering steps is maintained from 150° to 200° C.